Thus EP 0 930 979 B1 discloses a self-checking banknote which comprises a flexible plastic carrier. The flexible plastic carrier comprises a transparent material and is provided with a clouded sheathing which leaves a clear transparent surface free as a window.
A magnification lens is arranged in the window as a verification means. In addition provided on the banknote is a microprint region which manifests a small character, a fine line or a filigree pattern. Now, to check or inspect the banknote the banknote is folded and thus the transparent window and the microprint region are brought into overlapping relationship. The magnification lens can now be used to make the microprint visible to the viewer and thus verify the banknote.
Alternatively EP 0 930 979 B1 proposes arranging in the transparent window a distorting lens, an optical filter or a polarisation filter.